The electronic line switch fulfils a number of functions including the hook-switch function, that is, the line switch acts as a hook-switch when a hook-switch control signal is selectively applied to the line switch, a hook-switch control signal being generated when the user brings the subset into the off-hook mode. This control signal may be provided by a ten number repertory tone/pulse dialler chip or a microprocessor.
In a subset provided with a dialler chip and a conventional mechanical hook-switch, the dialler chip is provided with its operating power from current derived from the exchange battery and drawn over the exchange line through the hook-switch. As soon as the subset is brought into the off-hook mode the hook-switch contacts operate and adequate operating voltage is extended to the dialler chip which is then able to function.
In the case of a subset provided with an electronic line switch, however, when such a subset is initially connected to the exchange line, or reconnected after subsequently being un-plugged, the dialler chip is without power because its power source is cut off by the line switch which it controls. Consequently the dialler chip cannot function and the line switch cannot be signalled.
A known method by which the dialler chip of such a subset is provided with power is described in the specification of PCT/AU88/00046 (corresponding to commonly assigned U.S. application Ser. No. 07/377,830, now abandoned). This specification disclosed an arrangement wherein a storage capacitor provides a current source for the dialler chip. The arrangement is such that initially, with the capacitor discharged, upon connection of the subset's line terminals to the exchange line. Current flows via a circuit to cause a first transistor to switch on thereby rendering the subset's electronic line switch conducting. The capacitor is charged via the operated line switch until the voltage level across the capacitor reaches the minimum operation voltage of the dialler chip. A control circuit then switches off the first transistor and hence the line switch.
This known arrangement, however, is not satisfactory for providing power to the dialler chip in a lower voltage phone, and particularly low voltage phones in parallel, because of the low voltage at the phone line terminals.
Another known arrangement for providing power to the dialler chip is a bleed circuit around the line switch to bleed sufficient current from the exchange line in the on-hook mode. With low voltage subsets, particularly parallel low voltage subsets, the bleed current required to power the dialler chips would exceed the allowable on-hook current allowed by some telecommunication authorities.
A solution to the problem of providing power to the dialler chip in the on-hook mode is to provide the power with a dry cell. An undesirable feature of this solution, however, is the necessity to replace the cell when its capacity falls due to discharge. If the discharge is kept to a minimum, the cell life will be greatly extended.